The present invention relates to skid control systems, and more particularly relates to a skid control system including a novel control circuit.
It is an object of the present invention to provide a novel skid control system for controlling the brakes of the wheels of a wheeled vehicle.
It is another object of the present invention to provide a novel skid control system for fluid actuated brakes for wheels of a wheeled vehicle including a novel control circuit for actuating a modulating valve for modulating or controlling the pressure to the brakes in response to an electrical signal indicating the occurrence or the imminence of a wheel skid condition.
In the system of the present invention the modulating valve is actuated in response to an electrical signal indicating the occurrence of a preselected deceleration rate; in order to accommodate different surface conditions and provide optimum operational characteristics this rate is varied depending upon the surface condition which is sensed by sensing the spin up rate of the wheels being controlled during the time of brake relief. In addition, the time duration that the valve is operative to relieve pressure at the brakes of the controlled wheels is varied with road conditions and vehicle speed.
In the system of the present invention a vehicle speed analogue is provided which during each brake relief senses the linear velocity of the vehicle by sensing the velocity attained by the controlled wheels. This information is stored and is used for the next cycle to vary the time at which the brakes are reapplied in accordance with velocity. This provides for better high speed operation. The sensed velocity signal is allowed to change with time during each cycle at a preselected rate such that at any time an analogue approximating vehicle speed will be had. Two separate means are provided for determining when the brakes are to be reapplied. One means senses wheel spin up and is actuated at high spin rates, indicating high mu surfaces; the analogue is used to vary this occurrence with velocity such that at higher velocities the wheels can spin up more. The second means senses vehicle deceleration and reapplies the brakes after a determinable interval. This latter means is designed to be sensitive to low mu surfaces; in addition, the time for reapplying the brakes is increased with velocity in accordance with the velocity analogue.
When the system is used to control two wheels in unison, means are utilized to compensate for wheel brake unbalance which could permit one wheel to decelerate faster than the other, etc. Unless some compensation were provided a situation could occur in which one wheel was near vehicle speed while the other was in substantial slip; in such a condition most of the braking would be caused by only one wheel resulting in inefficient braking. To prevent this in the present system means are provided requiring the wheels to reach a minimum selected velocity before relief of the brakes. All of the above noted features contribute to an improved system and enhance the stability of the vehicle.
Therefore, it is another object of the present invention to provide a skid control system including one or more of the above noted features.